I fitted complete new valves to Aging Youths Humber for him a couple of years ago, as they were buggered.
They arrived with a circular patch of Hypalon already attached, so I just cut tightly around the old valve with a craft knife and stuck the new valve/patch over the hole.
It was a bit fiddly as the tube was obviously deflated, but it wasn't too bad.
This may not be the proper way to do it, but they held air perfectly at least up until he sold it a couple of months ago.
You can also just replace the inserts if you are careful. Do a search for Ribshops post about doing it with a piece of cotton as a safety line.
I did the valves this way on my boat following his method and it worked fine, but my valves were not that bad and did only need the inserts changed.
RIBBED For extra pleasure.
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